Study On Solidification Process Of High Performance Cast Iron Alloy

With the continuous improvement of the performance of the diesel engine, the key components,such as cylinder block and head, in combustion chamber of diesel engine, are subjected to higher thermal and mechanical loads, the important research direction about new head material is that the strong toughness and thermal conductivity of cylinder block and head material need to have better coordination.The strength-toughness and thermal conductivity of cast iron is closely related to the micro structure of graphite and matrix and the interface of graphite/matrix.Therefore, in order to improve the strength-toughness and thermal conductivity of cast iron, it is necessary to thoroughly investigate formation process and evolution mechanism of the microstructure of cast iron. A quantity of researches have been carried out in this aspect at home and abroad, but due to the limitation of the experimental conditions, it is basically a qualitative conclusion.In this paper, cast iron(HT300、RuT300、QT600),which was applied for domestic and international high-power diesel engine cylinder head,was studied. The sample of different stages of solidification were prepared by melt-quenching. The solidification process of cast iron were researched by thermal analysis. The microstructure and phase composition of material and the evolution mechanism of microstructure were researched by OM、 SEM、 EDS. Through DT2000 and microstructure analysis software, the nucleation and growth kinetics of graphite during three kinds of alloys solidification process was analyzed. Using X-ray 3D imaging technology, the growth process of graphite in gray cast iron was studied.Through the research on thermal analysis curves of three kinds of cast iron, the three species of cast iron had similar cooling curve that was divided into three stages.First stage, the cooling rate was fast in 5-60 s and was about 3.7 ℃/s. Second stage, the cooling rate was constant in 60-220 s. Third stage, the cooling rate was fast after 220 s and was about 3.6 ℃/s. The eutectic reaction time of compacted graphite cast iron was shortest and the reaction prosess was most intense. The eutectic reaction led to the liquid iron rise to about 18 ℃ and the maximum heating rate was about 0.8 ℃/s.The eutectic reaction time of ductile iron was longer and the temperature of liquid rose to about 4 ℃. The reaction time of gray cast iron was longest and the temperature was Basically unchanged.It was found that the final morphology of graphite was not related to the initial nucleation through the study on the solidification process of three kinds of cast iron, and the three kinds of alloy had the same initial nucleation mode that was the spherical precipitate nucleation. The graphite morphology of gray and compacted graphite cast iron was changed in the later stage of priority and the early stage of the eutectic. For gray cast iron, the growth of flake graphite was always connected to the liquid phase and relied directly to absorb carbon from the liquid. The graphite for nodular cast iron would be completely wrapped in liquid phase and the growth of it depend on carbon that diffuse into austenite from liquid iron. The growth behavior of graphite in compacted graphite cast iron was between gray iron and nodular cast iron.Through the study of compacted graphite cast iron nuclei found that the nuclei morphology of compacted graphite cast iron may include spherical, flake, irregularly shaped which be made up of spherical and flake as well as similar cylindrical shape. The graphite heterogeneous nuclei could be a single nuclei, but also could be a dual nuclei. The elements in graphite nuclei included C、Al、S、Ca、Ti、Ce、La、O, the ball nuclei contain CeO2、SiC、 CeS2、CaS、TiC、La2S3、Al2O3, and the size of nuclei was about 1.7～3.4 μm.It was found that the nucleation rate of the three kinds of cast iron first increased and then decreased though the study of nucleation and growth kinetics of graphite in three kinds of cast iron. The nucleation rate of gray cast iron graphite reached maximum of 40/mm2·s at 1148 ℃ and the eutectic stage was major growth period for graphite.The nucleation rate of nodular cast iron graphite got maximum of 400/mm2·s at 1180 ℃ and the primary stage was main growth period for graphite. The nucleation rate of compacted graphite cast iron graphite was maximum for 757/mm2·s at 1150 ℃ and the eutectic stage was mostly growth period for graphite.